fixes #2420 - extract strings for i18n from JavaScript, various i18n fixes

cc @lzap, @abenari for review

The main bits that are missing are:
* will_paginate (uses Rails i18n, was working on this separately)
* jquery datatable (jquery has its own i18n system that needs looking into)
* any other asset gems, e.g. spice-html5

I wanted to do this initially as a separate i18n resource/domain, but gettext_i18n_rails_js didn't support that, so this will have to do for now.  The resulting JSON files aren't that big anyway.